Ultrasound sensor for detecting the level of liquids

ABSTRACT

An ultrasonic liquid level detector includes a detector case ( 1 ), a rod acoustic waveguide ( 2 ) on one end of which there is an acoustical-electrical transducer ( 3 ) and on the other end a hollow resonator ( 4 ). The resonator ( 4 ) space is isolated from an external medium. The detector case ( 1 ) is fixed rigidly and hermetically on the surface of the rod acoustic waveguide ( 2 ) in a zone of minimum rod oscillations of the rod acoustic waveguide at operating detector frequency and contains an attachment means for attaching the detector to an external base.

APPLICATION FIELD

The invention relates to liquid level indicators using measurement ofsound waves parameters.

BACKGROUND OF THE INVENTION

Designs of ultrasonic liquid level detectors are known where resonatorsand tuning forks are driven with the help of sound waves. The acousticparameters of resonators and tuning forks vary when contacting liquid, amedium with higher density than air, and these variations are detected.

The design according to DE patent N 4201360 can serve as an example. Thedevice contains two or more vibratory rods put into a reservoir undercontrol, which are connected with radiating and receiving transducers.The same principle is used with the “Device for detection and/or controlof filled reservoir level,” according to DE patent N 4118793, or the“Device for measurement and/or keeping of given level in a reservoir,”according to the International application WO 92/21945.

The given designs of the liquid level detectors depend on operatingconditions. Liquid and dirt remaining in the tuning fork spaces caninfluence measurement accuracy of the mentioned devices.

A device for liquid level detection is known from DE patent N 3011603priority 26.03.80, Int. Cl. G 01 F 23/28. The device contains vibratoryelements placed co-axially. The space between the vibratory elements isisolated from the medium and is located along the whole length of therod. Piezoelements are fixed on membrane inserts connected to one of thevibrators. The case of the device is fixed to the membrane insert. Theattachment joint of the device case to the external basement is made inthe form of a threaded connection. The device differs from the offeredinvention in operating principle and design.

A further device for liquid level control is known from DE application N2949162 priority 06.12.79 Int. Cl. G 01 F 23/28. The device has a hollowportion along the whole length of a waveguide, to which a radiator andreceiver are fixed in different places. The case is not rigidly fixed tothe waveguide and includes a threaded connection for fixing it to theexternal base.

Moreover an acoustic liquid level detector is known from FR applicationN 2596515 priority 28.03.86 Int. Cl. G 01 F 23/28. The detector containsa hollow rod along the entire length and is open to the surroundingmedium. Another hollow pattern along the length of the waveguide isinstalled inside the rod on which transducers are installed. Thewaveguide attachment points to the external base are not considered inthis design.

An ultrasonic liquid level indicator is known from SU Invenitor'sCertificate N 231151, wherein the detector consists of two separatedwaveguides. The waveguides are fixed in the walls of a reservoir undercontrol of the oscillation nodes.

The designs of all above mentioned ultrasonic detectors differ from theoffered one. In particular, protection from the influence of condensate,any remaining liquid and dirt on measurement accuracy is not provided inthe above designs.

The “Detector of Liquid Presence” according to EP patent N 409732priority 19.07.90 Int. CL. G 01 F 23/28 is the closest in technicalprinciple. The design of the detector includes a case, a measuringelement connected with a pulse generator and a receiving device. Themeasuring element of this device includes an acoustical-electricaltransducer and a compound acoustic waveguide connected to thetransducer. The first part of the compound acoustic waveguide consistsof a solid cylinder and the second one consists of a hollow cylinder.The first part of the waveguide is smaller in diameter than the secondone.

The performance accuracy of the given detector design depends onconditions of the surrounding medium. Liquid and other contaminates canaccumulate in the open space. Liquid and dirt can also gather on thetransducer and on the waveguide case where the junction (transition) toa greater diameter occurs. All these can influence the accuracy of thedetector performance.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce detector performancedependence on a surrounding medium and various operating conditions,thereby providing high accuracy and sensitivity.

The ultrasonic liquid level detector according to the present inventioncontains a detector case and a rod acoustic waveguide on one end ofwhich there is an acoustical-electrical transducer and on the other—ahollow resonator. The acoustical-electrical transducer providesexcitation of the rod acoustic waveguide, which will oscillate onoperating frequency depending on the resonator. Oscillation duration ofthe excited resonator will depend on whether it is placed in a gaseousmedium or has contact with liquid.

The novelty consists of the following.

The resonator cavity is isolated from the external medium. Such a designavoids accumulation of liquid and foreign matter, and thus diminishesmeasurement errors. The resonator space is placed on the end opposite tothe acoustical-electrical transducer of the waveguide. The space isplaced where it is necessary to control the liquid level. This allows toreduction in the influence of dirt and condensate, because foreignmatter deposited on the waveguide part, which is not hollow, does notpractically influence measurement accuracy.

The detector case is fixed rigidly and hermetically on the surface ofthe rod acoustic waveguide in the zone of minimum rod oscillations ofthe rod acoustic waveguide at detector operating frequency. Such casefixing on the rod acoustic waveguide avoids ingress of moisture andforeign matter to the acoustical-electrical transducer and onto thewaveguide upper part. Therefore, operation accuracy and sensitivity ofthe detector are not reduced as a result of the rigid case being fixedto the rod acoustic waveguide, because it is fixed in the zone ofminimum oscillations of the waveguide rod.

The detector case contains an attachment means for attaching said caseto the external base. The location of the attachment means on the casealso serves as the solution of the set task. Since the mass on the case,even though it is rigidly and hermetically fixed on the acousticwaveguide, influences the detector parameters only minimally, the casefixed on the external base by means of the attachment means does notpractically influence detector parameters regardless of the base type.

In such a design there is no place on the rod acoustic waveguide whereliquid, condensate and dirt can gather. The case isolates entirely theacoustical-electrical transducer and the waveguide part from externalsurroundings. Moreover, the case does not practically influence thedetector parameters, regardless of where on whatever base the case isinstalled.

In a particular embodiment the resonator space is isolated from theexternal surrounding by a plate which is rigidly and hermetically fixedto the rod acoustic waveguide. The plate thickness is less than W/12,where W is the sound wavelength in the rod acoustic waveguide atoperating frequency. Such a plate thickness does not practicallyinfluence the detector sensitivity.

Besides, the case fixing zone on the surface of the rod acousticwaveguide should be placed in the nodes of longitudinal waveguideoscillations.

Such zone position under concrete values of detector operating frequencyprovides minimum case influence on detector parameters. For this purposethe distance between the fixing place and the transducer should be equalto an odd number of wavelengths quarters that with regard to acceptablespreads of this distance (equal to W/12) leads to the condition

W/4*[(2*k+1)+⅓]>L>W/4*[(2*k+1)−⅓],

where

W—sound wavelength in the rod acoustic waveguide at operating frequency;

k—integer value.

Additionally, the thickness of the detector case in the place of fixingon the rod acoustic waveguide is less than W/12, where W is the soundwave-length in the rod acoustic waveguide at operating frequency. It isnecessary to provide minimum case influence on detector sensitivity.

In addition, the attachment means of the detector case for attaching thecase to the external base is made in the form of a threaded connection.Such a means of attachment provides additional effects. For example,parasitic oscillations of the external base passed to the detector caseare absorbed to a large extent by the threaded connection, and are thenemitted in the form of heat energy. This provides detector performancewhich is substantially from external conditions.

Thus, all of features of the offered invention promote less dependenceof the detector performance on environment and operating conditions,without reduction of its measurement accuracy and sensitivity.

The main features set of the given design also provides additionaleffects.

Rigid and hermetic fixing of the detector case on the rod acousticwaveguide provider detector fire safety, because it isolates theacoustical-electrical transducer from the operating medium, the level ofwhich is measured by the detector. Therefore, fire hazardous liquids canform such a medium.

Another additional effect results due to the fact that the rigid fixingof the detector case on the rod acoustic waveguide does not influencethe detector performance only at the operating frequency. For acousticoscillations of other frequencies which may influence the detectoraccuracy, the case acts as an acoustic filter. Thus in the given design,the detector noise immunity increases. The plate isolating the resonatorspace of the rod acoustic waveguide plays the same role but to a lesserextent.

Since the waveguide is fixed to the case in the zone of oscillationsnodes, i.e. in the zone characterized by high acoustic impedance, theexternal actions lead to small oscillations in the waveguide atoperating frequency because acoustic impedance depends on frequency.Such fixing can be considered as an acoustic rejection filter atoperating frequency.

Positioning of the sensitive zone, the hollow resonator, at the end ofthe rod acoustic waveguide allows the detector to be placed arbitrarilyat the installation site. The rod acoustic waveguide can contact withliquid and the spatial arrangement of the detector does not practicallyinfluence the detector properties to determine the liquid level. Thecontact with liquid of the rod acoustic waveguide zone where the hollowresonator situates is important for measurement. That is why thedetector can be oriented arbitrarily; the main thing is that theresonator should be on the liquid indication level.

When indicating high-temperature and low-temperature liquids, theextended rod acoustic waveguide allows the placement of theacoustical-electrical transducer outside the zone with extremaltemperature and pressure.

The new features of the detector design according to the present providea detector with novel and patentable features.

BRIEF DESCRIPTION OF FIGURES

The invention is illustrated by the following figures:

FIG. 1 represents the detector design.

FIG. 2 represents fixing of the detector case to the rod acousticwaveguide.

FIG. 3 represents a variant of detector fixing on the base.

FIG. 4 represents time diagrams explaining the detector performance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detector design (FIG. 1) includes a rod acoustic waveguide 2 on oneend of which there is an acoustical-electrical transducer 3, on theother end is a hollow resonator 4. A detector case 1 is fixed rigidlyand hermetically on the surface of the rod acoustic waveguide 2 in thezone of minimum oscillations 6 of the rod acoustic waveguide 2 at thedetector operating frequency. The space 5 of the resonator 4 is isolatedfrom the external medium by the plate 8. The case 1 contains anattachment means 7 for attaching said case 1 to an external base. Theend of the rod acoustic waveguide 2 with the acoustical-electricaltransducer 3 and an electronic circuit board 9 with an impulse generatorcircuit and a circuit for detector signals processing are placed in thedetector case 1. The acoustical-electrical transducer 3 is connectedwith the board 9 by conductors 10. The detector board is connected to anexternal electrical circuit by a cable 11, which is introduced in thedetector case 1 through a hermetic seal 12. A hermetic cover 13 isolatesthe detector case space with the board 9 from the external medium.

The detector case 1 (FIG. 2) has a thickness of Lk with Lk<W/12 (W isthe sound wavelength in the rod acoustic waveguide 2 at operatingfrequency) at the location the case 1 is fixed on the rod acousticwaveguide 2. This joint(fixing) 14 can be of welded type.

The attachment means 7 for attaching said case 1 to the external base 15is made in the form of a threaded connection (FIG. 3).

The detector operates as follows.

The acoustical-electrical transducer 3 situated on one end of the rodacoustic waveguide 2 generates periodical oscillations in the waveguide,which have the form of impulse signals with sinusoidal attributes.Impulse signals are generated by an electronic generator situated on theboard 9. The signals propagate along the rod acoustic waveguide 2 andwhen they reach the hollow resonator 4 located on the opposite and ofthe acoustical-electrical transducer 3, oscillations in the resonator 4are produced. The resonator 4 oscillations are damped ones, whereby thedamping factor to a large extent depends on the properties of the mediumin which the resonator is placed. If the medium has a small waveresistance at the operating frequency then the damping factor is smalland oscillations damp slowly. If the medium in which the resonator isplaced has a resistance comparable to the resonator output resistance,as an acoustic radiator, then acoustic oscillations appear in the mediumand energy emissions from the resonator into the medium occur and aresufficient enough to increase the resonator damping factor. In this casethe resonator 4 oscillations damp rapidly.

The resonator 4 oscillations propagate from the rod acoustic wave guide2 towards the acoustical-electrical transducer 3, and when they reachthe transducer 3 they are converted into an electrical signal oscillates(FIG. 4). This signal repeats in the waveguide and are similar to theresonator oscillations. Thus, the electrical signal has a form of slowlydamped oscillations, i.e. oscillations which have a small damping factorif the resonator is placed in the medium with small resistance, forexample in a gaseous one (upper oscillogram in FIG. 4). And vice versa,when the resonator is placed in the medium with a resistance greaterthan a gaseous medium resistance, for example in liquid, the electricsignal has fast damped oscillations, i.e. oscillations with a greaterdamping factor (lower oscillogram in FIG. 4).

According to the damping factor, the detector signal processing circuitforms the output detector signal. This signal has a relay character andcarries information about the type of the medium in which the resonator4 is placed, namely whether the medium is liquid or gaseous. This signalis transmitted to the external circuit via the cable 11.

The detector space with the acoustical-electrical transducer 3 and theboard 9 is isolated from the external medium by means of the weldedjoint of the rod acoustic waveguide 2 and the case 1, seal 12 of thecable 11, and the hermetic cover 13. Therefore the medium in which thedetector case is placed does not influence operation of the electroniccircuit and the transducer.

The vibrations of the base to which the detector is fixed do not reachthe acoustical-electrical transducer and therefore they do not influencethe detector operation. This is as a result of the threaded connectionof the attachment means 7 which transmits the case oscillations to thedetector case 1 inefficiently (because of oscillations energy absorptionby friction surfaces of the threaded connection) as well as due to thelocation of the joint 14 in the zone of minimum oscillations of the rodacoustic waveguide 2. This zone is characterized by a significantresistance, which essentially prevents no oscillations from penetratingboth from the waveguide and into the waveguide.

The liquid level detector has an oscillatory system Q-factor, which isdetermined by the properties of the medium in which the sensitivedetector element—the hollow resonator—is placed.

Besides it is necessary that minimum rod oscillations where of thedetector case is fixed on the waveguide, regardless the mediumproperties in which the detector is situated.

For this purpose the dimensions of three main parts of the detector—thewaveguide, the resonator and the plate—should have certain wave relateddimensions.

It is convenient technologically, if the main parts of the indicatoroscillatory system (the waveguide, the resonator and the plate) are madefrom the same material and have the same external diameters.

The plate thickness Lp is chosen in such a way that it will have minimalaffect on the resonator Q-factor. For this purpose the plate thicknessshould meet the following condition

Lp<W/2π*arctg(α)  (1)

where α is the ratio of a wall squared cross-section of the resonatorand the waveguide. Preferably, Lp is less then W/12.

The hollow resonator length Lr and the waveguide length Lw are chosen insuch a way that the whole mechanical oscillatory system has a resonancefrequency equal to the operating one. Therefore, the resonance frequencydoes not depend on the wave resistance of the medium which contacts theresonator. Thus, the area of minimum waveguide oscillations does notdepend on the medium wave resistance. For this purpose it is necessarythat dimensions Lr and Lw meet the following equations:

Lw=(2*k+1)*W/4+Lp  (2)

Lr=W/2{n+1/π*arctg[2*α(1+α²)*tg(4πLp/W)]}  (3)

where k, n—integer values.

If the mechanical system meets these conditions then the areas ofminimum waveguide oscillations are located in distances equal to oddnumbers of W/4, from the waveguide end on which theacoustical-electrical transducer is fixed.

For example, when the external diameter is 12 mm, the resonator hollowpart diameter is 10 mm, and the plate thickness Lp=1 mm which issufficient for obtaining a firm construction. It follows from (1) thatthe operating frequency wavelength in the material should be more than21.2 mm. Let us choose the wave-length 36 mm, which for alloyed steelcorresponds to the operating frequency of about 140 kHz. The resonatorlength according to (3) is equal to 26.65 mm when n=1. The waveguidelength Lw from (2) will be 100 mm when k=5. Since the distance betweenthe zone of the waveguide fixed on the case and the transducer shall bean odd number of wavelength quarters, this distance L in this case maybe equal to 9 mm, 27 mm, 45 mm etc.

Industrial Applicability

The ultrasonic liquid level detector according to the invention can beused for level monitoring of different liquids including fire hazardousones. The detector can be installed on any bases, it is simple in designand adaptable to streamlined production. It has high sensitivity andsmall dependence on operating conditions, in particular waveguidecontamination.

High mechanical strength and absolute leak-proofness of the designallows the ultrasonic liquid level detector to be used in extremeconditions, including level indication of such products as liquefiedgases, e.g. liquefied air.

Since the rod acoustic waveguide of the detector can be made rather long(tens of centimeters), there is an opportunity to separate the sensitivezone, i.e. the resonator from the acoustical-electrical transducer andthe electrical circuit. Such separation possibility allows the detectorto be used for measurement of high-temperature liquids.

What is claimed is:
 1. An ultrasonic liquid level detector comprising adetector case, a rod acoustic waveguide on one end of which there is anacoustical-electrical transducer and on the other—a hollow resonator,wherein the resonator space is isolated from an external medium, and thedetector case is fixed rigidly and hermetically on the surface of therod acoustic waveguide in a zone of minimum rod oscillations of the rodacoustic waveguide at the detector operating frequency and contains anattachment means for attaching the detector case to an external base. 2.The detector according to claim 1, wherein the resonator space isisolated from the external medium by a plate which is rigidly andhermetically fixed to the rod acoustic waveguide, whereby the platethickness is less than W/12, where W is the sound wavelength in the rodacoustic waveguide at operating frequency.
 3. The detector according toclaim 1, wherein a zone at which the case is fixed on the surface of therod acoustic waveguide is located at a distance L from theacoustical-electrical transducer with W/4*(2*k+1)+⅓)>L>W/4*(2*k+1)−⅓),where W is a sound wavelength in the rod acoustic waveguide at operatingfrequency, and k is an integer value.
 4. The detector according to claim1, wherein a thickness of the detector case where it is fixed on the rodacoustic waveguide is less than W/12, where W is the sound wavelength inthe rod acoustic waveguide at operating frequency.
 5. The detectoraccording to claim 1, wherein said attachment means for attaching thedetector to the external base is a threaded connection member.